Help needed with sudden 120Hz hum in Mission Amps 5e3

[Justin Thomas]

@Dave H

I have checked the grounds with a meter, as you suggest

All are showing a connection to the chassis.

The hum is unchanged.

Are you using Resistance, or Continuity? Contonuity may not be the best feature in this case - usually anything up to 500ohms registers as "continuity." That's just not good enough for me when checking grounds...

Justin

[rdh006]

@Justin Thomas
Thanks for the reply. I am using Resistance. I will go and recheck for exact numbers. Can you tell me what setting you would use and what threshold you would find acceptable.

[rdh006]

@Justin Thomas
I initially set the meter to 2000 ohms and saw zeros.
Setting to 200 ohms, I see around 1 ohm, mostly 1.1, max 2.1

[Justin Thomas]

Hey rdh,

I was just trying to make sure you used the actual resistance setting; a lot of guys use continuity for ground, which can really wreak havoc with stuff like tyranny windings, artificial center taps for heaters, etc. No need for the actual numbers, I'd just make sure they're super-low, like over 10R and I get suspicious...

Justin

[rdh006]

Thanks, Justin.

[rdh006]

Here is a summary so far.

The hum was not present on Saturday, but was there at next power-up on Monday. The amp was working perfectly until something changed. (I live several miles out on an unmade road, and the amp has banged back and forth in a jeep for years -- I expected an easy-to-find broken connection...)

The hum appears to be 120 Hz. To my ear it sounds identical to 120 Hz from a tone generator, and quite different from 60 Hz -- although I have taken on board that it can be difficult to be certain.

Turning either volume to zero reduces the hum but does not completely remove it, (presumably because there is still some signal path due to the 5e3's interactive volume controls).
Turning both volumes to zero removes the hum.
Turning the tone pot to zero also removes the hum.

Pulling V1 alone -- hum remains.
Pulling V2 alone -- hum is reduced but still present.
Pulling both V1 and V2 -- hum is gone.

I have switched in a replacement tube in each position in turn. No difference.

Jumpering a new 22uF filter cap onto each of the existing 22uF filter caps made absolutely no difference. I understand Jazz P Bass' caution that this is not definitive, but I am still unsure how to make the measurements suggested.

I have measured and confirmed the 100-Ohm resistors in the filament artificial CT, and confirmed the connection to the power tube cathode with a meter.

I loosened all the input jacks and pots, checked/cleaned and re-tightened, and measured grounds for continuity to the chassis. I can confirm that they all show a good connection to chassis, and are all good and tight.

I have also checked the following grounds with a meter, and can confirm that they all show a good connection to chassis :
the filter cap -ve terminals
220k power tube grid resistors
power tube cathodes
preamp tube cathodes

Is there a path here, or any definitive elements? Can anybody see any narrowing down of options, or any probability?

[Chuck H]

It's almost certainly a bad ground connection. This could be an open lead wire connection, cold solder joint, a cracked component lead connection, a bad lug connection at a pot either to it's lead or at the wiper, a bad switch or jack connection or maybe even some other thing I'm not considering at the moment. "I" suspect that because there is still some hum with V2 removed that hum is impressing itself from V1 onto the power tube grids through the power supply rail and it's not being decoupled by the filter caps. That means a universal ground connection is open. I suspected the filter cap ground. You've tested that and confirmed it's good though. But test it again from the - filter terminals to the chassis, not the brass plate. If you get anything other than your meters "default" reading(*) test connection from the brass plate to the chassis.

Another possibility is... Well... There isn't one. With the amp still humming some with V2 removed, but not with both preamp tubes removed that can only mean the hum is coupling through the HV rail. And that can only mean the filters aren't decoupling the hum. And that can only mean they aren't properly grounded. Since the amp was fine, and then it wasn't with no work being done inside, that can only mean there is a faulted connection to ground. Putting it all together can only mean a bad HV rail filter ground.

I am prepared to be humble if I'm wrong, but I still think this is what it is regardless of your test result.

* The meter's default reading is what it reads with both probes touched together. It will rarely be zero. So whatever you read with the probes touching is the new zero.

[g1]

One other thing, the meter uses tiny current to test resistance. A connection can read fine and yet be unable to handle larger currents. It may go high resistance when passing any substantial current, but read low resistance on the meter.
This can be extremely frustrating but means a low resistance reading on the meter does not guarantee a reliable connection. Any good connection should read very close to 0 volts AC and DC across it when under power.

[Justin Thomas]

Is this an alternative way to check the integrity of a ground connection?

Learn one new thing each day!

Justin

[Jazz P Bass]

Kind of similar to reading voltage across a fuse under power.

If there is a reading, then the fuse is bad.

[rdh006]

@Chuck H
Thanks very much for your detailed analysis -- much appreciated.

[rdh006]

@g1
Thanks for another valuable piece of info.

[rdh006]

I just want to say at this point that I really appreciate the time everybody has put into trying to help, and the opportunity to learn from what has been said -- thanks, guys.

I haven't used my soldering gear in more than a year and, to rub salt in the wound, I have been unable to get it to work tonight -- I might need to order a new tip, so I may not be able to post a quick result from these suggestions. I will work through the grounds and re-make the connections as soon as I can, and post the result.

[g1]

If it's more than the tip and you need to fix the iron you should probably start a new thread for that.

[Chuck H]

What he said^^^^^^^^^^^^^^^^^^^^^^^^^^

And remember... Patience is a virtue (Grasshopper. Snatch the pebble... From my pants). As much as it sucks sometimes